One player suggests tweaking them in a little different way: the new Herald of the Titans achievement actually requires you to have a certain iLevel on your gear, so that you can't get it if you're geared out so well it makes the fight trivial. Players want to know why Blizzard can't do that with the Glory of the Raider achievements, so that no matter what new gear we get, the difficulty stays the same.
But Bornakk confirms that it's not a matter of difficulty, it's a question of rarity. If players were always able to get the mounts, there would always be more and more of the mounts in the game. They want to cut off the supply, period. The good news is that there will eventually be more rare mounts to go after (and I can't wait until I see that Gnome head flying around). But the proto-drakes' time is almost up.